Compound semiconductor devices using GaN or GaN series compound semiconductor are under active development. GaN has a wide band gap of 3.4 eV allowing a high voltage operation. Various types of semiconductor devices can be manufactured by forming a hetero junction of GaN series compound semiconductor. Metal organic chemical vapor deposition (MOCVD) is mainly used as a crystal growth method.
A semiconductor light emitting device using GaN series compound semiconductor can emit blue or ultraviolet light, and can form a white light source by using phosphors. Various light emitting devices are manufactured by growing GaN series compound semiconductor crystal on a sapphire substrate or an SiC substrate.
GaN has a high breakdown voltage, and is expected for applications in a field requiring a high voltage operation and a high speed operation such as high electron mobility transistors (HEMT) used in a mobile phone base station. Various types of GaN-HEMT have been reported having a GaN layer as an electron transfer layer in GaN/AlGaN crystal layers grown on a substrate such as sapphire, SiC, GaN and Si. A breakdown voltage value over 300 V in current-off state is presently reported. The best output characteristics are now obtained in GaN-HEMT using an SiC substrate. High thermal conductivity of SiC contributes to this performance. In manufacturing a high speed operation GaN device, a semi-insulating SiC substrate is used to suppress a parasitic capacitance.
International Publication WO00/04615 proposes to manufacture a semiconductor laser by forming AlGaN patterns having a (1-100) direction stripe shape on (0001) Si plane of an SiC substrate and growing GaN on the AlGaN patterns by MOCVD. The grown GaN layers gradually fill the gap between the AlGaN patterns.
JP-A-2003-309331 proposes to manufacture a semiconductor optical device or a semiconductor electronic device by forming a mask layer of amorphous insulator on a (0001) sapphire substrate, opening a square window through the mask layer to expose a part of the substrate and growing nitride semiconductor on the substrate.
JP-A-2002-53398 reports that single crystal GaN having a (0001) plane of hexagonal crystal structure with flat mirror surfaces was obtained by cleaning a 3C polycrystalline silicon carbide substrate oriented in a [111] axis direction at 1100° C., thereafter lowering a substrate temperature to 650° C. and growing GaN on the silicon carbide substrate by MOCVD using ammonium and trimethyl gallium as sources. This publication reports also that a silicon oxide film is laminated on a silicon carbide substrate, a circular window is formed through the silicon oxide film, and a GaN layer is grown thereon and that liquid phase epitaxy (LPE) is used in place of vapor phase deposition.
FIG. 10A is a schematic cross sectional view showing the structure of a GaN-HEMT device made in public by the present inventor. After an AlN buffer layer 103 is grown on a (0001) single crystal SiC substrate 101 by MOCVD, a non-doped GaN active layer 104, a non-doped AlGaN spacer layer 105 and an Si doped n-type AlGaN electron supply layer 106 are grown on the AlN buffer layer to form a GaN-HEMT structure, and then an n-type GaN protective layer 7 is grown. Formed on the n-type GaN protective layer 107 are a Schottky contact gate electrode G, an ohmic contact source electrode S and an ohmic contact drain electrode D. An exposed surface of the n-type GaN protective layer 107 is covered with an SiN film 108. Although GaN is hard to grow on the SiC surface, GaN can be grown more easily by forming the AlN buffer layer. A current collapse phenomenon that an on-resistance changes during operation can be avoided by forming the GaN protective layer and SiN layer on and above the n-type AlGaN electron supply layer. SiC has a high thermal conductivity and can realize a high speed operation at a high operation voltage. A price of a semi-insulating single crystal SiC substrate is high, leading a possibility of hindering to prevail GaN devices.
FIG. 10B is a cross sectional view of a SiC composite substrate. The composite substrate is formed by bonding a polycrystalline SiC substrate and an Si substrate. By using the polycrystalline SiC substrate, the cost is reduced and a high thermal conductivity of SiC is provided. By bonding the single crystal Si substrate, the single crystal characteristics can be retained. This SiC composite substrate is available from Soitech SA, France. An epitaxial layer is formed on the single crystal Si substrate. Si has a lower thermal conductivity than that of SiC so that the merit of a high thermal conductivity of SiC is difficult to be utilized fully. Since Si and SiC have different thermal expansion coefficients, a stress is generated.